Electrochemical double layer capacitors, also super- or ultracaps, use high porous electrodes, usually based on activated carbon. The partitioning of pore sizes into small, medium-sized, and large pores depends on the electrode material used and on the preparation of the electrodes (The Ultracapacitor Opportunity Report, Menahem Anderman, Advanced Automotive Batteries, 2005). The electrodes are separated by a so called separator, usually consisting of polypropylene or polyethylene. Paper or ceramic membranes may also be used. The electrolyte, which is mostly based on organic solvents, is placed between the electrodes, partially sucked by the separator and the electrodes. The electrolyte is the ionic conductive connection between the electrodes and it penetrates in between the single electrode particles.
Typical electrolyte systems used in today's supercapacitors are solutions of N(C2H5)4BF4 or N(CH3)(C2H5)3BF4 dissolved in acetonitrile or ethyl-methyl-imidazoliumBF4 in propylene carbonate. Owing to the extraordinary combination of low viscosity and high dipole moment of acetonitrile, these electrolytes have a very high conductivity. A typical electrolyte of 1.0 mol/l N(C2H5)4BF4 in acetonitrile shows an ion conductivity of almost 60 mS/cm at 25° C. Additionally, such electrolytes based on acetonitrile show superior characteristics at low temperatures and an acceptable voltage. Present capacitors are charged up to 2.5 V, partially up to 2.7 V.
Compared to modern lithium ion batteries, double layer capacitors can only accumulate a fractional part of energy. But they are capable of high current rates and they can be charged and discharged completely within seconds. Supercaps show additional advantages with regard to stability of life cycles (up to 1 million charge/discharge cycles compared to some hundred cycles for lithium ion batteries) and hence they seem to be interesting energy storage systems for a variety of stationary and mobile applications. Solely, their relative low energy density is regarded to be obstructive.